Thermal cycler for PCR

An apparatus for performing the polymerase chain reaction in a plurality of liquid reaction mixtures, said apparatus including a plurality of vials containing such liquid reaction mixtures, said vials having an upper portion and a lower portion, said apparatus comprising:

• an assembly for cycling said vials through a series of temperature excursions;
• a cover for applying a seating force to said vials and for applying a constant temperature to the upper portion of said vials;
• and
• a computing apparatus for controlling said temperature excursions of said assembly and said constant temperature of said cover;
• said assembly including:
• a sample block for receiving said vials;
• a plurality of thermoelectric devices controlled by said computing apparatus;
• a heat sink;
• a clamping mechanism positioned so as to clamp said thermoelectric devices between said sample block and heatsink;
• a heater positioned around the perimeter of said sample block;
• and
• a pin having a first end and a second end, said first end in close contact with said sample block and said second end in close contact with said heatsink so as to provide a thermal path between said sample block and said heatsink, said pin being positioned to reduce thermal gradients across said sample block.